1. Field of the Invention
The present invention relates to a photodetector made of a gallium nitride group compound semiconductor material and capable of detecting a light having a wavelength range about 390 nm to about 500 nm (hereinafter referred to as a gallium nitride group compound semiconductor photodetector).
2. Description of the Related Art
Photodiodes made of silicon, which can be integrated into a variety of circuits, are preferably used as photodetectors in the technological field of information equipment using light.
There is however the following problem with the silicon photodiode. An optical pickup uses light to read and write optical recording information or to perform servo control of these operations. Such light may have a wavelength of as small as about 600 nm or less in an attempt to achieve higher-density optical disks or the like. However, silicon has a low photosensitivity to the light having a wavelength of about 600 nm or less. The light detection peak wavelength (where the photosensitivity is maximized) of silicon is about 800 nm. The shorter the wavelength of light from about 800 nm, the greater the absorption coefficient of silicon, i.e., the more the loss due to absorption at a surface and surface recombination. For instance, the photosensitivity to light of about 600 nm is about 60% of the peak value; and 20% with respect to light of about 400 nm. Thus, the shorter the wavelength, the lower the photosensitivity. There has been a demand for a photodetector having a high photosensitivity to light having a short wavelength of about 600 nm or less.
Japanese Laid-Open Publication No. 7-288334 discloses a photodetector having a photosensitivity to light having a wavelength of 365-635 nm. This photodetector has a double-hetero structure in which an InGaN layer functions as a light detecting layer is provided between p-type and n-type gallium nitride group compound semiconductor layers.
FIG. 13 is a cross-sectional view illustrating the structure of a photodetector made of a gallium nitride group compound semiconductor material according to the above described conventional technology.
The conventional photodetector shown in FIG. 13 includes a buffer layer 132 of GaN, an n-type contact layer 133 of n-type GaN, an n-type cladding layer 134 of n-type Ga0.9Al0.1N, a light detecting layer 135 of In0.1Ga0.9N, a p-type cladding layer 136 of p-type Ga0.9Al0.1N, and a p-type contact layer 137 of p-type GaN, which are successively provided on a sapphire substrate 131. The photodetector further includes a Nixe2x80x94Au electrode (p-side electrode) 138 provided on the p-type GaN contact layer 137 and a Tixe2x80x94Al electrode (n-side electrode) 139 on an exposed surface of the n-type GaN contact layer 133. The thickness of the InGaN light detecting layer 135 is about 0.1 xcexcm.
A plurality of such photodetectors having the same structure shown in FIG. 13 but different light detection peak wavelengths were prepared by changing a molar fraction of In in the InGaN light detecting layer 135. The sensitivity of each photodetector was measured at its light detection peak wavelength. Results are shown in FIG. 14. FIG. 14 shows a relative photosensitivity where the photosensitivity to light having a wavelength of about 365 nm is regarded as 1.
As is seen from FIG. 14, a relative photosensitivity of about 0.38 was obtained with respect to a wavelength of about 380 nm; about 0.2 with respect to a wavelength of about 390 nm; and about 0.07 with respect to a wavelength of about 410 nm. The longer the light detection peak wavelength, the more significant the reduction in photosensitivity.
The light detection peak wavelength of the light detecting layer of InGaN can be controlled by adjusting the In molar fraction of the InGaN. A relatively small In molar fraction of InGaN causes the light detection peak wavelength to fall within the range of about 380 nm or less. To obtain a photodetector having a light detection peak wavelength of about 380 nm or more, the In molar fraction in the light detecting layer needs to be increased. It is however difficult to obtain an InGaN film having satisfactory crystallinity as well as a high In molar fraction. In the case of a thick film having a thickness of about 0.1 xcexcm, for instance, the more the In molar fraction (i.e., the longer the light detection peak wavelength), the more significant the degradation in crystallinity of the light detecting layer, resulting in an increase in dark current. This causes a significant reduction in the light detecting sensitivity with an increase in the light detection peak wavelength, which is shown in FIG. 14.
As described above, in the conventional photodetector including the InGaN light detecting layer, the longer the light detection peak wavelength, the more significant the reduction in photosensitivity.
A gallium nitride group compound semiconductor photodetector according to the present invention includes a substrate; and a multilayer structure provided on the substrate. The multilayer structure includes an n-type gallium nitride group compound semiconductor layer; a p-type gallium nitride group compound semiconductor layer; and a light detecting layer provided between the n-type gallium nitride group compound semiconductor layer and the p-type gallium nitride group compound semiconductor layer. The light detecting layer has a quantum well structure including a quantum well layer of InxGa1xe2x88x92xN (0 less than x less than 1); and a barrier layer of InyGa1xe2x88x92yxe2x88x92zAlzN (0xe2x89xa6y less than 1, 0xe2x89xa6zxe2x89xa61, y+z less than 1).
In one embodiment, the multilayer structure is provided in a direction parallel to the c-axis of the gallium nitride group compound semiconductor layers.
In one embodiment, the substrate is any one of a sapphire substrate, a SiC substrate, and a GaN substrate.
In one embodiment, the quantum well structure of the light detecting layer is a single-quantum well structure.
In one embodiment, the quantum well structure of the light detecting layer is a multi-quantum well structure.
In one embodiment, the number of quantum well layers included in the quantum well structure of the light detecting layer is in the range of about 1 to about 70.
In one embodiment, the width of each quantum well layer included in the quantum well structure of the light detecting layer is in the range of about 1 nm to about 15 nm.
Thus, the invention described herein makes possible the advantages of providing a photodetector having an improved photosensitivity to light having a wavelength in the range of about 390 nm to about 500 nm.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.